1. Field of the Invention
The present invention relates to an alternator driven by an internal combustion engine, and in particular, relates to a stator for an automotive alternator mounted in a vehicle such as a passenger car or truck.
2. Description of the Related Art
FIG. 8 is a sectional side elevation of a conventional automotive alternator. FIG. 9 is a front view of a stator core applied to a stator of a conventional automotive alternator. FIG. 10 is a circuit diagram of a conventional automotive alternator. The conventional automotive alternator shown in the figures includes, a case 3 constructed from an aluminum front bracket 1 and rear bracket 2, a shaft 5 with a pulley 4 fixed on one end thereof provided so as to be capable of rotating inside the case 3, a Lundell rotor 6 fixed to the shaft 5, fans 7 fixed on both sides of the rotor 6, a stator 8 fixed to an inner wall of the case 3, a slip-ring 9 fixed on the other end of the shaft 5 for supplying electric current to the rotor 6, a brush 10 for slidingly contacting the slip-ring 9, a brush holder 11 for receiving the brush 10, rectifiers 12 electrically connected to the stator 8 for rectifying an alternating current produced in the stator 8 into a direct current, a heat sink 13 fitted to the brush holder 11, and a regulator 14 attached to the heat sink 13 for adjusting the magnitude of the alternating voltage produced in the stator 8.
The rotor 6 includes a rotor coil 15 for flowing an electric current to generate magnetic flux and a pole core 16 which houses the rotor coil 15 and forms magnetic poles in accordance with magnetic flux. The pole core 16 includes, a pair of alternately meshed first pole core body 17 and second pole core body 18. The first pole core body 17 and second pole core body 18 are made of iron and have claw-shaped magnetic poles 19, 20 at end portions thereof. Gaps are formed between adjacent claw-shaped magnetic poles 19, 20 so that magnetic flux does not leak between the claw-shaped magnetic poles 19, 20, and also serve as cooling passages for cooling the rotor coil 15.
The stator 8 comprises a stator core 22 and two (2) sets of three-phase stator coils 23, i.e. a conductor wound on the stator core 22 at a phase difference of 30 degrees of electrical angle (see FIGS. 9 and 10). The stator core 22 is formed in an annular shape by punching thin steel sheets in a shape with convexo/concave portions at equidistant intervals and then lap winding or laminating the sheets. Slots 25 and teeth 24 extending in axial directions are formed in a inner portion of the stator core 22.
In the case of this example, there is provided the 2 sets of three-phase stator coils 23, there are twelve (12) magnetic poles of the rotor 6, 2×3 phases correspond to each pole, and seventy-two (72) slots 25 and teeth 24 are formed. The slots 25 are formed in the annular-shaped stator core 22 at a mechanical angle of 5 degrees (360 degrees/72). At this time, since 72 slots uniformly correspond to the 12 poles, the slots 25 are formed at a uniform interval of 30 degrees of electrical angle. The Y-Y connected two sets of three-phase stator coils 23 is provided at a phase difference of 30 degrees of electrical angle in each slot 25, and is electrically connected to the rectifier 12.
In the automotive alternator constructed in this manner, electric current is supplied from a battery (not shown) through the brushes 10 and the slip ring 9 to the rotor coil 15, generating magnetic flux. The claw-shaped magnetic poles 19 of the first pole core 17 are magnetized as north (N) poles by this magnetic flux, and those (poles 20) of the second pole core are magnetized as south (S) poles thereby. Meanwhile, rotational torque from the engine is transmitted through the belt and the pulley 4 to the shaft 5, rotating the rotor 6. Thus, a rotating magnetic field is applied to the stator winding 23, generating electromotive force therein. This alternating electromotive force passes through the rectifiers 12 and is converted into direct current, the magnitude of the current is adjusted by the regulator 14, and the battery is recharged.
In the above automotive alternator, there are one (1) slots 25 for each (1) set of the stator coils 23, each (1) phase and each (1) magnetic pole. Magnetic field leakage is seldom formed between adjacent claw-shaped magnetic poles 19, 20 of the rotor 6 via teeth 24 and the time over which magnetic flux leaks to the teeth 24 is short. Accordingly, there is a only a small decrease in the effective magnetic field for the stator coils 23 occurring due to magnetic flux leakage, and surges in the magnetic flux is reduced.
Moreover, an automotive alternator similar to the one described above is shown in Japanese Patent Application Laid-open No. 4-26345.
In a conventional automotive alternator constructed as above, since the stator core has 72 slots, which is a lot, there is a problem in that the insertion time and installation characteristics of the stator coils 23 in the stator core 22 are poor.
Also, with the conventional automotive alternator, as shown in FIG. 11 which is an electromagnetic field analysis chart drafted by the present inventors (Slot opening portions (the abscissa) are spaced a uniform interval of 30 degrees of electrical angle, this means that, for example, with a mechanical angle of 24 degrees, 24 degrees and 36 degrees are repeated and the interval is uneven. The ordinate shows ratios of higher harmonic components of magnetomotive force in the stator with respect to a fundamental wave) when the interval of the slot opening portions is an equal 30 degrees fifth (5th) and seventh (7th) harmonics are not present in the magnetic flux density waveform. Nevertheless, eleventh (11th) and thirteenth (13th) higher harmonic components of magnetomotive force in the stator 8 are large, and when 11th or 13th higher harmonic components of magnetomotive force are present in the rotor 6 there are problems in that, surges in magnetic flux cannot be sufficiently suppressed due to interference by the harmonic components and fluctuation in the generated voltage cannot be sufficiently suppressed. Moreover, magnetic attractive force is generated between the claw-shaped magnetic poles 19, 20 of the rotor 6 and the stator 8 and causes the stator 8, case 3 and the like or the claw-shaped magnetic poles 19, 20 of the rotor 6 resonate which generates a sound that is unpleasant for passengers.